WGBS of gill
Genomic DNA samples were fragmented to 200 - 300 bp using Covaris S220 sonicator (Covaris, MA, USA). Subsequently, DNA samples were treated with bisulfite according to the manufacturers instruction of EZ DNA Methylation-Gold kit (Zymo research, CA, USA). After bisulfite treatment, unmethylated cytosine became urancil, and methylated cytosine remains unchanged. After end repair and adenylation reaction, the bisulfite DNA fragments were subjected to PCR amplification to construct the sequencing library.